Metal recovery



Feb.4, 1936. HNGILBERT 2,029,998 METAL RECAOVRY Y Filedreb. 2,-195'3 Patented Feb'. 4, 1936 I' PATENT OFFICE METAL RECOVERY Harvey N. Gubert, man Fans, N. Y., signor 1 to The E. I. du Pont de Nemours a Company Incorporated, Wilmington, Del., a comltlon of Delaware applicati@ February z, 1933, serial No. 654,844

11 claim. (c1. 'z5-lss) a temperature below the freezing point of the Y This invention relates to the recovery of light metals from their salts or alloys and more particularly to the recovery of metal values from mixtures of light metals and non-metallic substances.-

The term light metals is used herein to designate the alkali metals, i. e. sodium, potassium,

lithium, etc., the alkaline earth metals, i. e. calcium, barium, strontium, and magnesium and beryllium. In the production of light metals by electrolysis of fused metal salts, the crude metal as obtained from the electrolytic cell usually contains va number of solid impurities, consisting mainly of metal salts and oxides. The crude metal is usually puried by mechanical separa upon the .nature of the-metal an'd the method of purification used, and in some cases may be .as much as Y90% by weight. The separation of any significant amount of metal from such residues heretofore has been exceedingly diilicult; and a complete separation of metal from the non-metallic substances present has been heretofore impossible. If a partial separation be eiected, for example by pressing, the nal residue still contains considerable amounts of metal and if the metal is of the more reactive type, for instance sodium, the residue is hazardoi's to handle and disposeof because of the danger of fire if it should come in contact with water or atmospheric moisture. Such residues can be disposed of only by destroying the metal, by vburning in a furnace or by other chemical means. When this is done, the resulting mixture of salts and oxides has no commercial value and must be wasted.

In some cases where a foreign salt is used to lower the melting point of the electrolyte in the production of a light metal by fused ksalt electrolysis, more or less of the-corresponding foreign metal also appears in the crude metal removed from the cell. For example, in the production of sodium by electrolysis of fused sodium chloride, calcium chloride is added to the electrolyte with the result that the crudesodium contains small amounts of calcium. In cases where the foreign metal is of limited solubility in the metal being produced, it may be removed from the liquid mixture along with the nonmetallic impurities by mechanical separation ,at

foreign metal. In such cases, the residue ob tained consists of considerable amounts of this foreign metal, mixed with the other salts and.

metals and oxides of one orboth of the metals.

An object of this invention is to substantially completely recover metal values from the aforesaid residues obtained in the separation of solid impurities from electrolytic light metals. A further obiectgs to eil'ect such recovery in such manner that non-metallic by-products of commercial value will be obtained. A still further object is to provide an improved means for reacting a light metal mixture or alloy with a salt or salt mixture to produce a different metal or alloy. Other objects will be hereinafter apparent. I have discovered that essentially complete separation-of metal from mixtures of the nature of the aforesaid residues may be accomplished by introducing the residues into a separating bath at a. temperature at which the metal to be recovered is liquid. The separating bath of my invention is a body of molten salt having a specific gravity-materially higher than` that of the metal. When such residues are added to the separating bath, the salts and oxides of the residues dissolve or settle out in the molten salt while the metal`or metals rise to the surface thereof, and may be drawn off in the liquid state or otherwise removed, thus eiecting a substantially complete separation of the metal from the non-metallic substances present. If it is'desired merely to eil'ect a mechanical separation of light metal from non-metals without changing the Vamount or nature of the metal or alloy originally present, the separating bath is made up of salts with which the metal residues will not react.

On the other hand, the composition of the fused salt separating bath may-be so selected that one or more metals present in the residue will react with one `or more constituents of the fused salt separating bath to produce another metal or alloy, this reaction being eilected simultaneously with the separation of the non-metallicconstituents.

One modication of my invention comprises contacting a residue containing more than one metal with a fused salt or salt mixture which reacts with ,all but one of the metals, so that the metal or alloy finally removed from the system is substantially free from the undesired metal or reacted with a salt of one of them. The meta-l whose salt is not originally present in the molten salt bath willreact by double decomposition with the salt to release the metal of the. salt in its stead, and the metal rising towards the surface of the salt bath will be substantially pure or will contain less of the metal kwhich reacts with the salt of the separating bath than was originally present, depending upon circumstances more fully described below.

I am aware that heretofore it has been proposed to produce light metals or alloys vthereof by reacting a light metal with a fused salt or salt mixture. However, in order to obtain light metals or alloys in good yields and of high degree of purity,

it has been necessary heretofore to perform these operations under such temperature conditions that the metal or alloy formed by the reaction is distilledol as it is formed. Such methods require relatively high temperature and more or less complicated apparatus unsuited to economical, large scale production. Without such distillation procedure, these processes heretofore have not been adopted for recovery of light metals from their mixtures with non-metallic compounds or for the separationof a substantially pure light metal from mixtures or alloys of light metals.

My invention is not to be confused with these prior processes wherein a light metal is produced by reacting another metal with a light metal salt. My process comprises the recovery of light metal values from a mixture of light metal or metals with non-metallic substances wherein a complete separation of the desired metal or alloy is effected in the liquid phase and preferably in a continuous manner. In its preferred form, my process enables substantially the same degree of separation obtainable by distillation methods but employs lower temperatures and simpler apparatus.

The appended drawing illustrates diagrammatically two forms of apparatus which may be utilized to carry out my invention.

Fig. l shows in cross. section a covered vessel I filled with a molten separating salt bath and heated by means not shown. Arranged within vessel I in such manner that it lies below the surface of the fused salt is a cylindrical, inverted bell 2. An inlet tube 3 passes through the. cover of the vessel I and thence through the top of the bell 2 to a point near but above Athe open end of the bell. An outlet tube 4 leads from the'top of the bell to the exterior of vessel I.

'Ihe apparatus illustrated by Fig. 2 is identical with that illustrated by Fig. 1 as respects the vessel I, the bell 2, the inlet tube 3 and the outlet tube 4, but has in addition a settling trap 5 connected to the outlet 4. Settling trap 5 is elongated in shape and has a downwardly extending member or sump 1 at the end opposite the connection to outlet 4. Sraping tool 6, which is inserted through the wall of trap 5- in such manner that it is capable of reciprocal motion and a certain angular displacement, is adapted to scrape solid material from the bottom oftrap 5 into sump 1. An opening, closed by cover 9, is situated in trap 5 directly above the sump 1. Outlet pipe 8 serves to carry liquid metal from trap 5.

One method of carrying out my invention may be described by reference to Fig. 1. A residue consisting of a sludgelike mixture of molten light metal or metals and insoluble non-metallic substances, e. g. saltsand; oxides, is introduced into the apparatus continuously through inlet 3.

At the bottom of inlet 3,1tlie.residue comes into .contain with-v molten salt, which dissolves certain of the non-metallic impurities; other impurities settle out, while metal substantially free from non-metallic substances collects in the upper portion of bell 2. Due to the diierence between the specic gravities of the salt andthe metal, the metal is forced upwards through outlet 4, where it may be collected in suitable containers, molds or the like. Preferably, the portion of the salt bath below bell `2 is lightly agitated, e. g. by a. current of gas inert to the metal being recovered,

so as tov prevent large amounts of the insoluble solid material from settling out in the bottom of vessel I; otherwise, there is a tendency for settled solids, which are heat insulators, to cause overheating of the bottom of the vessel. At intervals, accumulated solids may be allowed to settle out and then removed. The temperature of the molten bath of course must be above the melting points of the metals present and preferablyis maintained at such point that the viscosity of the bath is relatively low, in order that gravity separation will readily occur. The required difference between the specific gravities of the salt bath and the metal depends somewhat on the construction of the apparatus; for instance, when this difference is small the height to which the outlet tube 4 rises above the salt bath level must be correspondingly shortened.

In some cases, as mentioned above, the residues entering the process will contain more than one free light metal. The method of handling such residues according to my invention where it is desired to obtain only one of the metals present, is illustrated by the following example:

Example The process is carried out in the apparatus diagrammatically illustrated by Fig. 2 of the drawing. Vessel I is a covered steel cylindrical pot, for example, feet deep by 4 feet in diameter. The bell 2 is 24 inches in diameter by 4 feet 6 inches long, the top of the bell being situated about 17 inches below the cover of vessel I. Inlet 3 is a 4 inch iron pipe 6 feet 6 inches in length, extending to within 18 inches of the bottom of the bell 2. Settling trap 5 is a cylindrical vessel 16 inches in diameter by 7 feet 3 inches in length, the downwardly extending member 'I being 16 inches in diameter by about 12 inchesv in length.

Vessel I is mounted in a furnace 'heated by a gas flame and is filled to a point above the top of bell 2 with a fused, anhydrous mixture of calcium chloride and sodium chloride, containing between 60 and 75% by weight of sodium chloride. -metallic sodium, about 20% of metallic calcium, and about 10% of salts and oxides of these metals, is continuously fed into the apparatus by way of inlet 3, while the salt bath is mildly agitated by a stream of nitrogen introduced near the bottom of vessel I by means not shown. As the residue comes' into contact with the molten salt, part of the non-metallic constituents dissolve in the bath, theremainder, which is chiefly calcium oxide; tends to settle in the -lower portion of the bath; the calcium reacts with the salt bath as follows:

At the start of the operation, the temperature of the bath in vessel I is maintained at between 700 and 800 C.; later, as the sodium chloride content of the bath decreases, dueto the reaction between calcium and sodium chloride, the

A llter residue, containing about 70% of melting point of the salt mixture is correspondingly lowered and the temperature may be reduced to around 600 C. During the process, samples of the molten bath are removed from vessel I from time to time and analyzed to determine the sodiumv chloride and calcium oxide contents. The sodium chloride content of vthe bath is maintained at not lessv than 30% by weight by withdrawing portions of the bath and flowing sodium. Trap is air-cooled, preferably by blowing a blast of air against the exterior surface, to cool the metal therein to a temperature of 100 to 175 C. Preferably, the temperature in trap 5 is maintained so that the temperature of the sodium leaving at exit pipe 8 is between 150 and` 175. At these temperatures, the calcium precipitates and settles out in the bottom of .the trap. At regular intervals of time, for instance every fteen minutes, the calcium settling out in the bottom of the trap is scraped into the sump 1 by means of the scraper 8. From time to time, the precipitated calcium is removed from sump 1 by a hailing device inserted into the apparatus by way of the opening closed by cover 9. The material thus removed consists of a mixture of sodium and calcium, containing a small amount of oxide, and may be recycled through the processl by introducing it by way of inlet 3. The sodium issuing from the apparatus by way of outlet B is substantially free from oxide and/.or

` salt and may be substantially free from calcium or may contain up to 1% of calcium, depending on how eiliciently the processl has been carried out. Samples of sodium obtained by this process from the above residue the following compositions:

Sample l Sample 2 Sample 3 Percent Percent -Percent Sodium 99.81 99.94 99.968 T otal calcium (lree and combined) A as Ca 0. i8 0. 05 0. m6 Chlorldes, as Cl 0. 0l 0.01 0.006

kali and/or valkaline earth metal salts is elec-l trolyzed and the crude metal thus obtained is purified by mechanically separating solid impurities from the liquid metal, e. g. by filtration.4

The residue obtained'by the purication step, consisting of av mixture of metals, and non-metallic substances is reacted with a fused salt. ac-

cording to the above described method in such manner that the desired metal, alloy or metal mixture is obtained. The salt mixture result ing from this reaction in the separating bath is used to replenish electrolyte in the electrolysis step. Thus the cycle is complete with only some alkaline earth oxides to be disposed of after removal from the separating bath.V

The electrolysis of sodium chloride to produce substantially pure sodium is one example of this method of practicing my invention. In this case, the electrolyte may consist of a mixture of sodium chloride and calcium chloride. The crude metal removed from the electrolytic cell contains varying amounts of metallic calcium and metal salts and oxides. The liquid crude metal is filtered at a temperature at which calcium is least soluble therein, to produce sodium having a pu-I rity of more than 99%. The filter residue consists of a mixture of sodium and calcium, together with around 10% 'of salts and oxides. This residue'is introduced into a fused separating bath comprising calcium chloride and sodium chloride as described in the above example. The calcium reacts with the sodium chloride of the separating melt (to-produce calcium chloride'and sodium the non-metallic constituents of the residue are sepmated and the sodium originally present is recovered, together with that formed by the reaction, in a state of high purity. The calcium chloride produced by the reaction is recovered, for example, as a mixture of sodium chloride and 60-70% by weight of calcium chloride; this recovered salt mixture, together with more sodium chloride is used to replenish the electrolyte in the electrolytic cells. In this method of operation the calcium goes through a continuous cycle, in which it appears successively as calcium chloride and as the free metal. There is of coursev a certain loss of calcium, due chiefly to oxide formation; this small loss may be made up by adding fresh calcium chloride to the electrolytic cells as required.

'I'his cyclic process affords a means of utilizing the calcium produced along with the sodium during electrolysis and has other advantages. Ordinarily, when it is desired to add calcium ohioride to the fused electrolyte in the electrolytic production of sodium from sodium chloride, the calcium chloride first must be thoroughly dehydrated to prevent explosions due to the formation of steam within the 'molten electrolyte. The salt mixture recovered in the above described process contains from 60% to 70% calcium chloride, is substantially anhydrous and hence may be added to the electrolytic cell bath in lieu of pure calcium chloride. Furthermore, because of the presence of the relatively large amounts of sodium chloride, the mixture is less hygroscopic pure calcium chloride; consequently, there is less danger of absorption oi water by the calcium chloride before themixture can be introduced into a cell bath than when pure calcium chloride is used.

Obviously, many variations of the above described process may be made without departing from the spirit and scope of my invention. For example, separation of the metal issuing from outlet 4 after cooling may be effected by means other than settling, forv instance by filtration. Furthermore, my invention is 'not restricted to the continuous process illustrated above. For instance, filter residues or other mixtures of light metals or lightl metals and non-metallic substances may be mixed with a suitable molten separatingsalt and the mixture allowed to settle and cool in amold out of contact with air or moisture. .The-resulting molded mass will consist essentially of a solidified metal layer and a salt layer which can -be mechanically separated.

issuing from the process may be correspondingly varied. For example, by varying the calcium chloride content of the salt bath described in the above example, sodium-calcium alloys of various compositions may be produced. If it is desired to produce an alloy rich in calcium, the calcium chloride content of the bath is maintained at a high value, e. g. more than ri% by weight, whereupon the following reaction will occur:

The process advantageously can be used to recover a substantially pure light metal, from a mixture, not only by completing the separation in a settling trap or other mechanical means as illustrated by the above example, but by effecting a complete separation by means of reaction with the moltensalt separating bath, utilizing a series of runs if necessary. For example, a mixture of sodium and calcium, either pure or contaminated with non-metallic substances, may be reacted With a salt mixture rich in a calcium salt to produce an alloy of sodium and calcium, the major portion of which is calcium. This alloy may be again reacted With a salt bath containing large amounts of calcium chloride and, if necessary, the process' repeated a. third time to produce calcium substantially free from sodium. In like manner, the process may be used to produce other metals and alloys having low melting points.

The temperature at which the process is carried out depends upon the respective melting points of the salts and metals involved in the process, and also to some extent upon the relative viscosity of the molten salts and metals at various temperatures above their melting point. 'I'hat is,

the metals and salts present must be suiliciently uid at the operating temperature to permit effective gravity separation of the bath components.

Preferably the fused salt bath is maintained substantially free from water, especially if highly reactive metals such as sodium are present. However, small amounts of water may be present, provided the resulting hydrogen evolution is not excessive,

My process is usefuland advantageous in many respects. It permits efficient, economical, and effective separation of light metals` from each other or from non-metallic impurities, and effects such separations `more efficiently and -economivcally than has heretofore been possible.

gravity greater than that of sodium at a temperature of 600800v C., said bath comprising calcium chloride and not less than 30% by weight of sodium chloride. Y

, 2. A process for recovering sodium from a mixture of sodium, calcium, and non-metallic substances comprising introducing said mixture into a fused salt separating bath A having a specific l*gravity Agreater than that of sodium, at a temperature of 600-800 C., said bath -comprising calcium chloride and not less than 30% by weight of sodium chloride, and collecting within said bath and removing therefrom liquid sodium substantially free from non-metallic substances.'y

3. A process ,for recovering an alkali metal from a mixture of light metals comprising introducing said mixture into a fused salt separating bath having a specific gravity greater than that of the light metal to be recovered at a temperature below the boiling point of the metal to be recovered, said bath comprising suillcient amounts of a salt of the alkali metal to be recovered to cause said salt to react with at least one constituent of said light metal mixture and collecting within said` bath and removing therefrom a liquid mixture of light metals.

4. A process for recovering sodium from a mixture of sodium, calcium, and non-metallic substances comprising introducing said mixture into a .fused salt separating bath at a temperature of 600-800 C., said bath comprising calcium chloride and not less than 30% by Weight of sodium chloride, collecting within said bath and removing therefrom a liquid mixture of sodium and calcium substantially free from non-metallic substances, cooling said collected liquid mixture to precipitate calcium and separating said precipitated calcium.

5. A process for recovering alkali metal values from a mixture of an alkali metal, an alkaline earth metal, and non-metallic substances which comprises introducing said mixture into a fused salt separating bath containing not less than 30% by weight of a fusible salt of said alkali metal and having a specific gravity greater than that of the alkali metal to be recovered, at Aa temperature below the boiling point of the metal to be recovered whereby saidalkaline earth metal reacts with said alkali metal salt to liberate alkali- 6. A process for recovering an alkali metal from a mixture containing alkali metal, alkaline earth metal, and non-metallic materials, comprising introducing said mixture into a fused salt separating bath having aspeciflc gravity greater than that of the alkali metal to be recovered at a temperature below the boiling point of said alkali metal to be recovered, said bath comprising sufficient amounts/of a salt of the alkali metal to be recovered to cause said salt to react with alkaline earth metal present in said mixture, thereby releasing the desired alkali metalin metallic form.

7. A process for recovering sodium from a mixture of sodium, calcium, and noli-metallic substances comprising introducing said mixture into a fused lsalt separating bath 'having a specific gravity greater than that of sodium at a temperature below the boiling point of sodium, said bath containing atleast 30% by weight of a fusible sodium salt, whereby the calcium reacts with said sodium salt to liberate metallic sodium and collecting within said bath and removing therefrom liquid sodium substantially free from non-metallic substances.

8. A process for recovering sodium from a mixture of sodium, calcium, and non-metallic substances 'which comprises introducing said mixture into a fused salt separating bath having a specific gravity greater than that of sodium at a temperature below the boiling point of sodium,

said bath comprising sufficient amounts of a sodium salt to cause said salt to react with calcium in said mixture to liberate metallic sodium and then collecting in said bath and removing therefrom a liquid mixture of sodium and small amounts of calcium.

9. A process for recovering sodium from a mixture of sodium, calcium, and non-metallic substances which comprises introducing said mixture into a fused salt separating bath having a specic gravity greater than that of sodium at a temperature below the boiling point of sodium, said bath comprising suiiicient amounts of a sodium salt to cause said salt to react with the calcium present in said mixture and then collecting within said bath and removing therefrom substantially pure liquid sodium.

10. A process for recovering sodium from a mixture of sodium, calcium, and non-metallic substances comprising introducing said mixture into a fused salt separating bath at a temperature below the boiling point of the sodium, said bath comprising calcium chloride and not less than 30% by weight of sodium chloride, collecting within said bath and removing therefrom a liquid mixture of sodium and calcium substantially free from non-metallic substances, cooling said collected liquid mixture to precipitate calcium and separating said precipitated calcium.

11. A process for recovering alkali metal values from a mixture of an alkali metal, an alkaline earth metal, and non-metallic substances which comprises introducing said mixture into a fused salt separating bath containing not less than 30% by weight of a fusible salt of said alkali metal and having a specific gravity greater than that of the alkali metal to be recovered, at a temperature below the boiling point of a metal to be recovered, whereby said alkaline earth metal reacts with said alkali metal salt to liberate alkali metal, collecting within said bath and removing therefrom said alkali metal in liquid state containing a small proportion of alkaline earth metal and substantially free from non-metallic substances, cooling said collected liquid metal to precipitate a metallic constituent and separating the precipitated constituent.

, HARVEY N. GILBERT. 

